Many processes have been experimented and put into service in the past, i.e., in France (1938), Brazil (1881), Australia (1865), China (1881), Scotland (1862), Spain (1822), Italy (1937 Ing, F. Roma process, Italian Pat. No. 329457), South Africa (1935), Sweden (1938), USSR (1922) and in U.S.A. before the year 1858.
With the exclusion of the "in situ" processes, i.e. with underground heating and combusion, all the "surface processes" are based on the use of retorts. The most important available processes at the present time are the so-called "Development Engineering Inc. -Paraho process", the "Tosco II", the "Union Oil", the "Petrosix", the "Institue of Gas Technology IGT", "Hytort" (U.S. Pat. Nos. 4,003,821; 3,891,403; 3,992,295; 3,929,615; 3,703,052), the "Lurgi Ruhrgas", "NTU-" (U.S. Pat. Nos. 1,469,678; 1,536,696), and the so-called "Circular Grate Process" (U.S. Pat. Nos. 4,058,905 and 4,082,645).
The process could be classified in processes "solid-to-solid", in which the heat is transferred to the shale oil by means of balls (Tosco II), heated intert materials, spent shale or coke (Lurgi) or in processes with internal or external heating. The internal heating processes envisage the partial combustion into a retort of the oil or gas products of the shale or tar sands, i.e. the processes Bureau of Mines, Paraho Developement Engineering Inc., Union Oil. In the external heating processes the gas for the process is heated outside the retort by means of surface heat exchangers as in the Union Oil, IGT and Petrosix processes. This last process is very similar to the mentioned Ing, F. Roma process with the only difference that the Roma patent envisaged two condensers having the purpose of recovering the heat of the processes gases and condensing the oil product in one of the condensers.
All the mentioned processes and the others not yet industrially proved envisage costly and large heat exchangers, necessary for the condensation of oil products and/or heating the process gas, provided with heat transfer surfaces, which are difficult to operate and to maintain, costly and not always efficient equipments for dust depuration such as cyclones and electrostatic precipitators and moreover envisage vertical retorts or rotating drum or grates or a sealed screw conveyor (Lurgi), which cause a high pressure drop and therefore a high energy absorption for the circulation of gases.